An Al Qaeda leader, the head of a designated terror organization and a confessed jihadist-in-training are among a "Who's Who" of controversial figures who have participated in weekly prayer sessions on Capitol Hill since the 2001 terror attacks, an investigation by FoxNews.com reveals.

The Congressional Muslim Staff Association (CMSA) has held weekly Friday Jummah prayers for more than a decade, and guest preachers are often invited to lead the service. The group held prayers informally for about eight years before gaining official status in 2006 under the sponsorship of Rep. Keith Ellison, D-Minn., one of two Muslims currently serving in Congress. The second Muslim congressman, Rep. Andre Carson, D-Ind., joined as co-sponsor after he was elected in 2008.

Among those who FoxNews.com determined have attended the prayer services during the Clinton, George W. Bush and Obama administrations are:

— Anwar al-Awlaki, the notorious Al Qaeda cleric believed to be hiding in Yemen and the lone American on the U.S. government’s capture or kill list, who conducted a prayer service on Capitol Hill shortly after the Sept. 11 terrorist attacks.

— Randall “Ismail” Royer, a former communications associate for the Council on American-Islamic Relations (CAIR), who confessed in 2004 to receiving jihadist training in Pakistan. He is serving a 20-year prison term.

— Anwar Hajjaj, former president of Taibah International Aid Association, which was designated a terrorist organization by the U.S. and U.N. in 2004.

— Esam Omeish, the former president of the Muslim American Society, who was forced to resign from the Virginia Commission on Immigration in 2007 after calling for "the jihad way," among other remarks.

— Salam Al-Marayati, president of the Muslim Public Affairs Council, who was forced to step down from a national terrorism committee post in 1999 for pro-terrorist comments.

— Nihad Awad, CAIR executive director, who attended a Hamas meeting in Philadelphia in 1993 that was wiretapped by the FBI.

— Johari Abdul Malik, Dar al-Hijrah imam, who made statements in support of convicted and suspected terrorists who attended his mosque.

— Tariq Ramadan, a Muslim scholar banned from the U.S. for six years beginning in 2004 for his alleged ties and donations to terror groups. Secretary of State Hillary Clinton lifted Ramadan's ban in January.

— Abdulaziz Othman Al-Twaijri, the head of a division of the Organization of the Islamic Conference, considered a foreign agent by the U.S.